Optical discs are known as data recording media based on an optical principle. In addition to discs of the VLP (video long play) type already in wide use exclusively for reproduction, various optical discs have been developed which include those of the WORM (write once read mostly) type adapted for wrinting only once, and optomagnetic discs adapted for erasure and rewriting.
FIG. 14 shows an optical head device for projecting a light beam on an optical disc 26 to record data thereon or reproduce the recorded data. The laser beam B emitted by a laser light source is focused on the recording layer 27 of the disc 26 by an objective lens 97, which is disposed out of contact with the disc 26 and shiftable axially thereof by a focus actuator 65. The lens 97 is so controlled for focusing that a beam spot P having a specified diameter at all times will be formed on the recording layer 27.
Examined Japanese Patent Publications SHO 53-37722 and SHO 60-30017 disclose such focus control system and focus actuator.
On the other hand, optical tapes are under investigation as tapelike recording media having a greater capacity to optically record data than optical discs. Unexamined Japanese Patent Publications SHO 57-64333 and SHO 62-149036 disclose optical head devices for recording data on an optical tape or reproducing the recorded data.
With reference to FIG. 13, the disclosed optical head devices comprise a rotary drum 1 for passing an optical tape 2 around its outer periphery over a predetermined angular range. The drum 1 is drivingly rotated by a drive motor 12. Disposed above the rotary drum 1 is an optical system 9 including a beam producing-detecting assembly, i.e., a laser diode 91, polarization beam splitter 92 and photosensor 93, and a collimator lens 94. Fixedly provided in the drum 1 centrally thereof is a reflecting prism 95 for reflecting the beam from the assembly toward a beam emanating aperture 15 formed in the peripheral wall of the drum 1. A convergent lens 96 for focusing the beam reflected from the prism 95 on the optical tape 2 is disposed in the aperture 15 a specified distance inwardly away from the outer peripheral surface of the drum 1.
With the rotation of the drum 1 driven by the motor 12, the tape 2 is caused to travel along the outer peripheral surface of the drum 1 by the operation of an unillustrated tape transport mechanism and is thereby helically scanned with the laser beam from the optical system 9, whereby data is recorded or reproduced.
However, the optical head device conventionally used for the optical tape 2 serving as a recording medium has the problem that minute dust particles adhering to the tape 2 cause recording or reproduction errors. This is due to the reason that since the beam spot on the tape has a very small diameter, for example, of about 1 micrometer, the dust particle blocks the beam if adhering to the tape.
With the optical head device of FIG. 14 for the optical disc serving as a recording medium, a transparent base layer 28 formed on the surface of the disc recording layer 27 has a relatively large thickness (e.g. 1 to 2 mm , so that the diameter of the beam spot formed on the surface of the base layer 28 is, for example, as large as 1 mm. Accordingly, dust particles of about 10 micrometers in size, even if adhering to the surface of the base layer 28, will cause no trouble to the recording or reproduction operation. On the other hand, with the optical tape which needs to be highly flexible, a transparent base layer (21, in FIG. 7), when covering the surface of the recording layer (22, in FIG. 7), must be as thin as about 10 micrometers, for example. Accordingly, the diameter of the beam spot on the surface of the base layer is also as small as tens of micrometers, so that the adhesion of minute dust particles poses a problem.
Further when the optical head device of FIG. 13 is to be equipped with the focus actuator disclosed in the foregoing publication SHO 60-30017, the focus actuator must invariably be provided inside the rotary drum 1 along with the convergent lens 96 and therefore requires a rotary drum of increased size, hence a problem. Moreover, since the focus acturator is adapted to position the objective lens in place by a magnetic force and a spring force in balance therewith, the provision of the means therefor gives rise to another problem in that the vibration due to rotation produces focus control errors.